The present invention relates generally to a cryogenic cooling system for a synchronous machine having a rotor with a high temperature super-conducting (HTS) component. More particularly, the present invention relates to a cooling system to provide cryogenic fluid to the rotor and to re-cool used cooling fluid returned from the rotor.
Super-conductive rotors have their super-conducting coils cooled by liquid helium, with the used helium being returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power. Accordingly, there is a need for a cryogenic cooling system that reliquefies the hot, used cooling fluid returned from the rotor. The reliquefied cooling fluid should then be available for re-use as an HTS rotor cooling fluid.
High temperature super-conducting generators require highly-reliable, low cost cryorefrigeration equipment in order to be viable as commercial products. Redundant cryorefrigerator components have in the past been used to achieve high reliability with existing cryorefrigeration equipment. The inadequate reliability of these components and the requirement that HTS rotors have an uninterrupted supply of cooling fluid have in the past necessitated that redundant components be included in cryorefrigeration systems for HTS rotors.
The cost of cryorefrigeration systems is substantially increased due to the need for redundant cryorefrigerator components. Moreover, existing cryorefrigeration systems require frequent maintenance due to their inadequate reliability and system redundancies. Accordingly, the operating cost of these systems is relatively high.
Typical cryorefrigerator equipment for the temperature range of 20-30xc2x0 Kelvin is based on Gifford McMahon coldhead technology that has limited refrigerator capacity and requires maintenance about once a year. Multiple units can be combined to increase the capacity and reliability of the system at the expense of increased cost. In addition to multiple coldheads, closed loop circulation systems of cryogen gas require either cold re-circulation fans, or external warm re-circulation fans with counter-flow highly-efficient heat exchangers. These components add cost and complexity to the system when redundancy for high reliability is required, unless all components can be built with six sigma quality.
The purchase and operating costs of existing cryorefrigeration systems significantly add to the cost of machines having HTS rotors. These high costs have contributed to the heretofore-commercial impracticalities of incorporating HTS rotors into commercially-marketable synchronous machines. Accordingly, there is a substantial and previously un-met need for cryorefrigeration systems that are less expensive, inexpensive to operate and provide a reliable supply of cryogenic cooling fluid to an HTS rotor.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron forging.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate an air-gap magnetic field strength of about 2 Tesla. Known super-conducting rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher, which increase the power density of the electrical machine and result in significant reduction in weight and size. Air-core super-conducting rotors, however, require large amounts of super-conducting wire, which adds to the number of coils required, the complexity of the coil supports, and the cost.
A cryogen gas re-circulation cooling system has been developed for a High Temperature Super-conducting (HTS) rotor. This cooling system generally comprises a re-circulation compressor, a counter-flow heat exchanger, and a cooling coil heat exchanger inside a liquid cryogen storage tank. Cooling fluid flows from the re-circulation compressor through the heat exchanger and coiling cool (where the fluid is cooled to cryogenic temperatures) and then to the rotor and its super-conducting coil. Used cooling fluid is returned from the rotor, through the counter-flow heat exchanger (where heat from the compressed cooling fluid passing to the rotor is transferred to the used gas) and back to the re-circulation compressor.
The liquid cryogen in the storage tank is cooled by a re-condenser cryorefrigerator. The recondenser cryorefrigerator may be a single stage Gifford-McMahon (GM) cryocooler, a pulse tube with separate or integral compressor with a re-condenser unit attached to the single stage, or other such cryogen cooling system. The liquid cryogen may be nitrogen, neon, or hydrogen. Similarly, the cryogen gas in the re-circulation system may be helium, hydrogen, neon, or nitrogen.
The cooling system provides a steady supply of cooling fluid to an HTS rotor. Moreover, the cooling system is economical in its construction and operation. The reliability and economy of the cooling system facilitates the development of a commercially viable synchronous machine with an HTS rotor.
In a first embodiment, the invention is a cooling fluid system for providing cryogenic cooling fluid to a high temperature super-conducting machine comprising: a re-circulation compressor; a storage tank having a second cryogenic fluid; an inlet line connecting the re-circulation compressor to the storage tank and to the rotor, and forming a passage for cooling fluid to pass from the re-circulation compressors through the storage tank and to the machine.
In another embodiment, the invention is a cooling fluid system coupled to a high temperature super-conducting rotor for a synchronous machine, said system and a source of cryogenic cooling fluid comprising: a re-circulation compressor; a cryogenic storage tank storing a supply of cryogenic fluid; an inlet line providing a fluid passage for cooling fluid between the re-circulation compressor and the rotor, wherein the inlet line passes through the storage tank, and a return line providing a fluid passage for the cooling fluid between the rotor and re-circulation compressor.
In a further embodiment, the invention is a method for cooling a super-conducting machine using a cooling fluid system having a cooling fluid circuit, a cryogenic storage tank, a heat exchanger and inlet and return lines for cooling fluid, said method comprising the steps of:
a. pumping the cryogenic cooling fluid through inlet line, through the heat exchanger, the storage tank and into the machine;
b. transferring heat from the cooling fluid in the inlet line at the heat exchanger and into the return line, where the inlet and return lines pass through the heat exchanger;
c. cooling the cooling fluid to cryogenic temperature in the storage tank, and
d. returning used cooling fluid from the machine, through the return line and back to the inlet line.